Electric discharge device



March.29, 1938. R. M. soMERs 2,112,719

ELECTRIC DSCHARGE DEVICE Filed Nov. l0, 1934 ATTORNEY Patented Mar. 29,1938 UNITED STATES 2,112,719 ELECTRIC DISCHARGE DEVICE Richard M.Somers, Orange, N. J., assignor to Thomas A. Edison, Incorporated, WestOrange, N. J., a corporation of New Jersey Application November 1o,1934, serial No. '152,401

Claims.

This invention relates to electric' discharge devices of thetypethroughout the normal operation ol which a small quantity ofincluded metal is vaporized, providing within the device a metal I vaporat low pressure through which an arc discharge takes place. tion relatesto those low pressure, metal vapor discharge devices which also containan inert gas, or combination of gases, the gas providing a medium for atemporary or initial arc discharge whose 'function is to heat the devicefrom an initially cold condition to substantial metal Vaporizingtemperature. thereto, the invention is particularly applicable "Y todischarge devices having appreciable discharge column lengths, andespecially to illuminating discharge tubes. A

It is an object of my invention to provide an improved device of thetype above outlined which tion from the current supply.

Still another object of my invention is to provide a device of the typeoutlined in which the temporary arc discharge will be dependablyproduced at any current sup'ply voltage within sub-.

stantially the full range o f supply voltages appropriate to themaintenance of the normal vapor arc discharge.

Other and allied objects will more fully appear from the followingdescription and the appended claims.

In the description reference is had to the accompanying drawing, ofwhich:

Figure 1 is an illustration of a typical discharge device of the typeabove outlined, together with an appropriate associated circuit throughwhich the device may be connected to the current supply;

Figure 2 is a typical curve of a function herein- 45 after described,plotted against gas pressure; and

Figure 3 is a curve of optimum gas pressure, as hereinafter explained,plotted against discharge column length.

Before outlining my invention proper I refer to 50 and describevindetail the typical system illustrated in the ligure. The dischargedevice has been illustrated as one adapted for illuminating purposes,having two anodes, and arranged for operation on an alternating currentsupply; it 55 will hereinafter be obvious, however, that my in- Moreparticularly my invenv While not in al1 aspects limited acterized by avery high efficiency-i. e., ratiooi'- power in the discharge column topower consump- Vention is equally available for employment with otherforms of discharge devices and current supplies, such for example assingle anode devices and vdirect current supplies, or double elementtubes for alternating current operation in which each element isalternately anode and cathode. In the figure will be seen the tube lhaving the glass or other envelope 2, the space 2' within the envelopehaving been evacuated of air and containing an inert gas, orcombination' of gases, at pressures as hereinafter more fully set forth;the gas may for example be krypton. Also within the space 2 is provideda small amount of the metal whose vapor is to form the medium for thenormal arc discharge; and while I intend no limitation to any specificmetal, the small pool 2", within the space 2 in the ligure may beconsidered as a pool of mercury. 'I'he elements of the tube are thecathode 3, the anodes 4a and 4b, and the auxiliary starting electrode 5.The cathode 3 is of the thermionic type, and by way ofexample has beenillustrated as a folded filament which may be coated with suitableoxides or the like to increaseits emission. The anodes 4a and lb arelocated near the opposite end of the tube from the' cathode 3, and mayfor example lbe of the vusual carbon variety. The starting electrode 5may be a conductive ring surrounding and in slight spaced relationshipto the cathode 3.

The discharge current for the device may be obtained from an alternatingcurrent line through the transformer 6,'which by way of illustrationhas' been shown-as an auto-transformer. One side of the line isconnected through the on-off switch 9' to the primary terminal 6p of thetransformer, while the -other side of the line is connected selectivelyto the primary terminalsI 6p', 6p and Gp'" through the selecting switch92. 'I'he transformer may have the extreme secondary terminals 6a and 6band the secondary center-tap 6c; the first two are connectedrespectively to the anodes la and 4b through the ballast resistances orlamps 8a and 8b. while the center-tap 6c is connected to Athe cathode 3through the inductance coil or choke I0 both the lamp 8a-8b and thechoke I 0 form ballast orcurrent-limiting impedance means for thesystem.4 A tertiary winding 'l forming a part of the transformer 6 maybe connected across the extremities of -the filament or cathode 3 forheating the same. For starting thel temporary arc discharge through thegas I have illustrated herein, though purely by way of example, thestarting system shown inthe Figure 3 of U. S. Patent 2,013,974 tomyself, which system is described in that patent with reference to quitesimilar designating numerals. With this system the armature I2" isattracted by choke l0 to open the closelybiased switch I2, and to breakthe circuit I0-6c-6d-l2-53, when the cathode 3 has attainedsubstantially normal operating temperature; a high voltage transient isthus produced across the choke and causes the initiation of thetemporary arc discharge. This-discharge heats the device, graduallyvaporizing more and more metal; and the arc discharge transfers itself(usually gradually) from the gas as a medium to the vapor as a medium,forming the normal vapor are discharge.

It is well understood that to maintain the normal vapor are discharge ina given system of the type described the supply voltage must bemaintained above a minimum limit. It is also true, neglecting the purelytheoretical case of a perfect ballast, that there is a maximum valuewhich the supply voltage must not be permitted to exceed. This upperlimit of supply voltage appears to be occasioned as follows: With anincrease of supply voltage .there will occur some increase of dischargecurrent and hence of temperature within the device. This temperatureincrease causes vaporization of more of the included metal (e. g., 2"),increasing the vapor pressure, and occasions an increase of drop in thedischarge column. As the supply voltage and hence the temperatureincrease, not only does the discharge column drop rise, but the rate ofits rise progressively increases; presently a temperature is reached atwhich the drop tends to exceed the value which the supply voltage canmaintain between anodes and cathode, and extinction of the dischargeresults. Thus the vapor arc discharge when in progress will bemaintained only so long as the supply voltage remains within a definiterange; this may lconveniently be termed the vapor voltage range and itslimits respectively the upper and lower vapor limiting voltages.

As to the temporary arc discharge through the gas, there is in generalno significant upper supply voltage limit. There is, however, a lowerpermissible supply voltage limit which may be termed the gas extinctionvoltage; if the supply voltage falls below this limit while thetemporary arc discharge is in progress,l extinction will result. For thelow vapor pressures the lower vapor limiting voltage is'always exceededby the gas extinction voltage. Accordingly the system, if it is to beused without the requirement for manipulation of its parameters duringoperation, has a net permissible supply voltage range whose upper limitis the upper vapor limiting voltage, but whose lower limit is the gasextinction voltage.

It will of course be understood that the absolute values of the limitsof both the vapor and the net permissible voltage ranges may be togetheradjusted upwardly or downwardly, as by changing of the transformer ratioby adjustment of tap-selecting switch 9"; and that the most importantcharacteristic of either range is the ratio of its upper to its lowerlimit, or its logarithmic width. For a given ballast, this ratio orwidth may be increased (as to both vapor and net permissible voltageranges) by shortening the discharge column; again, for a given tube orother device this ratio or width may be increased (as to both ranges) byincreasing the ballast of the system. In either case the ratio or widthincrease, representing increased tolerance to supply voltagefluctuation, is obtained by increase of the relative ballast`-i. e., byincrease of the power in the ballast relative to that in the dischargecolumn. It is common practice to impart to the system a sufciently widenet permissible voltage range for operation with any particularv rangeof supply voltage uctuation..

simply by a suflleient progressive increase of this relative ballast.But for conventional discharge devices with any ballast the gasextinction voltage is materially higher than the lower vapor limitingvoltage, and the width of net permissible voltage range therefore verymucl less than that of the vapor voltage range; accordingly in commonpractice a relative ballast is employed which provides a width of vaporvoltage range very much greater than will be needed for supply voltagefluctuationsA during continuance of the 'vapor arc discharge, and awholly useless tolerance thus obtains during this continuance, i. c.,throughout the actually useful operation of the system.

According to my invention I employ a relative ballast suilcient to makethe vapor voltage range only a little wider than the range of supplyvoltage iiuctuations, and yet maintainv the net permissible voltagerange just as wide as the supply range by reducing to a very low valuethe excess of gas extinction voltage over lower vapor limiting voltage.By this procedure I am able to secure dependable operation withmaterially higher than usual eiliciencies. The manner in which I reducethe difference above mentioned may be outlined as follows:-

If for a device and system of the type described there be plotted theexcess of gas extinction voltage over lower vapor limiting voltage as afunction of the pressure of the included gas, it will pressure thefunction attains a minimum value; the curve of such a function has beenillustrated as A in Figure 2, taken in connection with the left-hand setof ordinates. While for devices otherwise similar different gasesv willcause the corresponding functions to have their respective minima atdiierent gas pressures, I have' found that for any particular gas thelvpressure at which the function minimum occurs is essentiallyindependent of ballast parameters and of parameters of the dischargedevice properotber than the length of the discharge column` oranodecathode separation. I have further found that the gas pressure forminimum function value is for practical purposes similar to the gaspressure for minimum gas extinction voltage: been indicated in Figure 2by the right-hand set of ordinatesapproximate gas extinctionvoltageslikewise applying to the curve A. According to my invention Iestablish the gas within the device at a pressure which causes thefunction above mentioned to assume substantially its minimum value, or,as a satisfactory approximation, at a value which renders the gasextinction voltage a substantial minimum for example at the pressurewhich is the abseissa of the point P on curve A.

I have further found that the function (or extinction voltage) willremain invested with a minimum value upon variation of the dischargecolumn length if the gas pressure be varied substantially inversely withthis length. Therefore, in convenient terms, I establish the gaspressure P at a value given by the expression P K/L wherein L is thelength of the discharge column in convenient units) and K is a constantdebe found that at some particular gas.

this has pressure in atmospheres for the gas krypton,

plotted against discharge column length in centimeters. It will beunderstood that this curve B represents the locus of the minimum point Pof a family of the curves A of Figure 2, expressed in terms of pressurefor various column lengths (the locus of the point P, expressed in termsof excess voltage for various lengths, exhibiting a variation, fromconstancy of that excess, which is small enough to be disregarded).

The constant K for any gas is readily determined by adjusting that gasin a representative discharge device to the pressure yielding theminimum value of the function above discussed, and multiplying thatpressure by the discharge column length in that tube. Because the gaspressure for minimum function value is approximated by the pressure forminimum'gas extinction voltage, a satisfactory constant for practicalpurposes may be arrived at by adjusting the gas in a representativedischarge device to the pressure yielding minimum gas extinctionvoltage, and multiplying that pressure by the` discharge column length.In the latter case the representative device may be a simple dischargedevicei. e., one from which vaporizable metall has been omitted. Y

I may illustrate the application of my invention by an example. Letthere rst be assumed a mercury-containing tube intended for operationwith a mean discharge current of approximately 2 amperes and at amercury pressure of approximately .000133 atmosphere, the tube -having adischarge column length of 100 centimeters and a diameter of 2centimeters. Let this tube be assumed lled with krypton at a pressure ofapproximately .00133 atmosphere, which is repre-'- sentative ofvconventional practise. With asuitable ballast, the vapor are dischargemay be maintained in this tube by a fluctuating supply voltage whichmaintains between terminal 6c and either of the terminals 6a. and'sb avoltage ranging from 80 to 115 volts; the lower and upper vapor limitingvoltages are accordingly 80 and 115 respectively. The gas extinctionvoltage, however, will be of the order of 100 volts, so that the netpermissible voltage range is only 100 to 115 volts. If the system is tofunction on a cur rent supply .whose voltage fiuctuates between limitshaving the ratio oi" 115:100, it is obvious that neither can the ballast-be decreased, nor can the 4tube be lengthened. The excess of gasextinction voltage over lower vapor limiting voltage is approximately 20volts, and is quite typical.

I have found' that by the application of my invention to such a tube Imay decrease the excess of gas extinction voltage over lower vaporlimiting voltage to between 2 and 5 volts, the precise value within thissmall range depending on many factors such as normal tube operatingtemperature, room temperature, and associated circuit parameters. Usingthe maximum difference figure of 5 volts, I may lengthen the dischargecolumn to such a length that the vapor voltage' range is 95 to 115volts, while still maintaining the net permissible voltage range at theassigned 100 to 115 volt value. Since the tube at a voltage just aboveits original vapor limiting voltage would operate with an arc drop ofvery approximately .4 volt per centimeter, I may lengthen i thedischarge column by or 37.5 centimeters. In round figures I may increasethe discharge column length by IA-i. e.,

from 100 to 133 centimeters. A lower magnitude of ballast impedance willnow serve to keep the upper limiting voltage at 115 volts. With thereduced ballast impedance the current through the lengthened tube, forany supply voltage throughout the 100 to 115 volt range, will not be fardifferent from the current at that supply voltage through the originaltube and ballast. Since the voltage drop in the discharge column isincreased by 33%, an eilciency increase of very nearly 33% is obtained.

'I'he lengthening of the tube is rendered permissible by the employmentof the proper gas pressure. To determine this pressure for thelengthened tube I divide .4, the constant for krypton above mentioned,by 133, the length of the revised discharge column in centimeters, the'quotient of approximately .003 being the krypton pressure in atmosphereswhich I employ for the lengthened tube. It will of course be obviousthat instead of lengthening the tube in this example I might, uponestablishment of the optimum gas y pressure of .004 atmosphere for thetube with its original length, have reduced both the ballast and themean supply voltage. 'Ihe same result of eiciency increase withoutimpairment of dependability of operation would be secured.

It will be understood that the voltages to which I have referred herein,except as otherwise spe- 'cially qualified, are voltages measured acrossthe primary of the transformer 6, to whichv voltages there isproportional the sum of the voltages across the discharge device (I) andvacross `the ballasting means (I0 and 8a).

It will be understood that the broader aspects of my invention are notintended to be limited by the examples set forth herein, but that thescope of the invention is intended to be expressed in y the followingclaims:

and thermionic cathode elements spaced apart to provide a substantialpositive column space therebetween, and containing lkrypton at apressure ,of approximately .4/L atmosphere, L being the anode-cathodeseparation in centimeters.

2. A low pressure metal vapor discharge device including anode andthermionic cathode elements spaced apart to provide a'substantialpositive column space therebetween, and containing krypton at a pressureof approximately .4/L atmospheres, L being the anode-cathode separationin centimeters. A

3. A low pressure mercury vapor discharge device including anode andthermionic cathode elements spaced apart to provide a substantialpositive column space therebetween, and containing krypton at a pressureof approximately .4/L atmosphere, L being the anode-cathode separationin centimeters.

4. An electric discharge device including anode and thermionic cathodeelements spaced apart to provide a substantial positive column spacetherebetween; ballast means adapted for serial connection with saidelements to a current source;

between; ballast means said elements and forming therewithv a loadcirvaporizable metal within said device for providing a low pressurevapor discharge medium; and an initial heating discharge medium withinsaid device, comprising krypton at a pressure of approximately .4/Latmosphere, L being the anodecathode separation in centimeters.

5. An electric discharge device including anode and thermionic cathodeelements spaced apart to provide a substantial positive column spacetherebetween; ballast means adapted for serial connection with saidelements to a current source; vaporizable mercury within said device forproviding'a low pressure mercury vapor discharge medium; and an initialheating discharge me- 'dium within said device, comprising krypton at apressure of approximately .4/L atmosphere, L being the anode-cathodeseparation in centimeters.

6. An electric discharge device including anode and thermionic cathodeelements spaced apart to provide a substantial positive column spacetherebetween; ballast means adapted for serial connection with saidelements to a current source; vaporizable metal within said device forproviding a low pressure vapor discharge medium; and an initial heatingdischarge medium included in said device, comprising an inert gas at apressure adjusted for substantially minimum gas discharge extinctionvoltage.

'ls An electric discharge device including anode and thermionic cathodeelements spaced apart to provide a substantial positive column spacethereserially lconnected with cuit; means for applying voltages acrosssaid circuit; vaporizable metal within said device for providing, whenhot and with voltages across said circuit above a lowerlimitirig4voltage, a low pressure vapor discharge medium between said elements;and an inert gas included in said device at a pressure adjusted toprovide a gas discharge medium between said elements with substantially8. An electric discharge device including anode and thermionic cathodeelements spaced apart to provide a substantial positive column spacetherebetween; ballast means serially connected with said elements andforming therewith a load circuit; means for applying voltages acrosssaid circult; vaporizable metal within said device for providing, whenhot and with voltages across said load circuit above a lower limitingvoltage, a low pressure vapor discharge medium between said elements;and an inert gas included in said device at a. pressure adjusted -toprovide between said elements a gas discharge medium characterized by anextinction voltage, measured across said load circuit, of within vevolts above said lower limiting voltage.

9. An electric discharge device including anode and thermionic cathodeelements spaced apart to provide a substantial positive column spacedtherebetween; ballast means adapted for serial connection with saidelements to a current source; vaporizable m'etal within said device forproviding a low pressure vapor discharge medium; and an initial heatingdischarge medium included in said device, comprising an inert gas at apressure of approximately K/L, L being the anode-cathode separation insaid device, and K being the product of anode-cathode separation and gaspressure in a simple purely gas discharge device containing similar gasat pressure adjusted for minimum discharge extinction voltage.

l0. A low pressure metal vapor discharge device including anode andthermionic cathode elements spaced apart to provide a substantialpositive column space therebetween and containing an inert gas at apressure of approximately K/L, L being the anode-cathode separation insaid device, and K being the product of anode-cathode separation and gaspressure in a simple purely gas discharge device containing similar gasat pressure adjusted for minimum discharge extinction voltage.

' RICHARD M. SOMERS.

